Precursory Patterns, Evolution and Physical Interpretation of the 2025 Santorini-Amorgos Seismic Sequence

Davide Zaccagnino, Georgios Michas, Luciano Telesca, Filippos Vallianatos

The 2025 Santorini-Amorgos seismic sequence marked a significant episode of volcanic-seismic unrest in the Hellenic Volcanic Arc, offering a unique opportunity to investigate precursory patterns and the dynamic evolution of seismicity in a complex tectonic setting. Here, we analyze the preparatory phase of the crisis using a high-resolution relocated seismic catalog, anomaly detection, and statistical modelling. We identify four distinct stages of seismic activity: 1) An initial volcano-driven phase starting in the summer 2024 with slightly accelerating moment release and focusing towards the Amorgos region; it was followed by 2) a progressive onset of the seismic sequence during January associated with stronger clustering, steady b-value and rocketing magnitude entropy with right-shifting multifractal spectrum. 3) A successive very energetic transitional, five-days-long chaotic phase in early February, with evident breakdown of the Gutenberg-Richter law, apparent decrease of the b-value, multifractality and hypocenters super-diffusion. Finally, 4) a terminal sub-diffusive aftershocks-dominated phase occurring after mid-February. Clustering, fractal and entropy analyses reveal significant premonitory changes marked by progressive migration of seismicity towards the area that hosted the sequence. Our findings suggest that the 2025 sequence was promoted by multiscale crustal weakening processes likely triggered by a magmatic-tectonic interaction and governed by the strong segmentation of the Santorini-Amorgos normal-faulting system preventing the strike of a mainshock with magnitude larger than 6.